The invention relates generally to monitoring electrical sensors and more specifically to a method and apparatus for predicting the failure of a resistive-type automotive sensor.
Resistive sensors are used in many applications on automobiles. Manual lever position sensors, fuel injection pump levers, exhaust gas recirculation sensors, and throttle position sensors are frequently of the resistive type. By example, the throttle position sensor, typically mounted on the engine, provides feedback to the engine control module in the form of an analog signal proportional to the position of the throttle. The throttle position sensor is typically a potentiometer-type sensor having a thin-film resistive coating on a substrate. A wiper is mechanically coupled to the throttle and sweeps the resistive coating. The wiper position corresponds to the relative throttle position such that a change in the throttle position effectuates a corresponding change in the wiper voltage.
In operation, the wiper utilizes certain portions of the sensor more regularly than others (e.g., the cruising position or idle). Consequently, the wiper thins the resistive coating in such areas and eventually may wear the coating off the substrate. If a sensor reading is taken from a worn portion of the coating, noise is imposed on the throttle position sensor signal. When the sensor first starts to wear, only small noise spikes are imposed on the signal and a change in the performance of the powertrain (i.e., the engine and the transmission) is imperceivable by the vehicle operator. However, over time, the magnitude of the noise spikes generated will increase, resulting in the degradation of the performance of the powertrain. For example, noise spikes may cause an electronic transmission to prematurely shift gears.
One method for discriminating between noisy and noise-free signals is described in U.S. Pat. No. 4,336,593. A discriminator is used to determine if a sensor signal is outside a predetermined range. The signals are not passed to the register or the microcomputer until they are within the predetermined range. Because only selected values are sent to the processing unit, a response delay is created in the system. It is believed that if such a system were implemented to process the throttle position sensor signal, there would be a noticeable degradation in the system performance since signals which correspond to worn portions of the sensor would not be passed until a non-noisy signal value was received. The result would be a sudden step in the throttle voltage. Also, the system will pass noise that falls within the acceptable range. Over time the sensor and powertrain performance would continue to deteriorate. Therefore, such a system is not desirable for use in detecting throttle position sensor wear.
It is desirable to detect the degradation of a throttle position sensor before damage to the powertrain occurs and the performance of the powertrain is reduced to a point where it can be perceived by the vehicle operator, thereby allowing corrective measures at the earliest possible time.
Since the underhood environment is inherently noisy, it is also be desirable to discriminate between underhood electrical noise and noise from the worn sensor.